A prior art scroll compressor, or pump, 100 is shown in FIG. 10. The pump 100 comprises a pump housing 102 and a drive shaft 104 having an eccentric shaft portion 106. The shaft 104 is driven by a motor 108 and the eccentric shaft portion is connected to an orbiting scroll 110 so that during use rotation of the shaft imparts an orbiting motion to the orbiting scroll relative to a fixed scroll 112 for pumping fluid along a fluid flow path between a pump inlet 114 and pump outlet 116 of the compressor.
The fixed scroll 112 comprises a scroll wall 118 which extends perpendicularly to a generally circular base plate 120. The orbiting scroll 122 comprises a scroll wall 124 which extends perpendicularly to a generally circular base plate 126. The orbiting scroll wall 124 co-operates, or meshes, with the fixed scroll wall 118 during orbiting movement of the orbiting scroll. Relative orbital movement of the scrolls causes a volume of gas to be trapped between the scrolls and pumped from the inlet to the outlet.
A scroll pump is typically a dry pump and not lubricated. In order to prevent back leakage, the space between the axial ends of a scroll wall of one scroll and the base plate of the other scroll is sealed by a tip seal 128. An enlarged cross-section through a portion of the fixed scroll 112 showing the tip seal 128 in more detail is shown in FIG. 11.
As shown in FIG. 11, the tip seal 128, typically made from a plastics material or rubber, is located in a channel 132 at the axial end 134 of the fixed scroll wall 118. There is a small axial gap between an axial end of the tip seal 128 and the base of the channel 132 so that in use fluid occupying the gap forces the tip seal axially towards the base plate 126 of the orbiting scroll. Accordingly, the tip seal is supported on a cushion of fluid which serves to urge the seal against an opposing scroll.
When bedding in or during use, the tip seals 128 are worn by contact with the opposing scroll base plate 120, 126 generating tip seal dust. When the pump is used for pumping a clean environment such as a vacuum chamber of a silicon wafer processing apparatus, it is desirable that the tip seal dust does not migrate upstream into the vacuum chamber, particularly during pump down times.